Adverbs

[amoeba]

I wonder if someone might be kind enough to explain to me the use of Latvian adverbs.

My grammar gives the following examples for adverbial use:

Labi, ka tu man rakstīji
(It's good that you wrote to me)
Vagonā bija neērti.
(It was uncomfortable in the carriage)
Ārā ir auksts.
(It's cold outside)
Āfrikā ir ļoti karsts.
(It's very hot in Africa)

Why is the adverb used in the first two examples (labi, neērti), but the adjective in the third and fourth examples (auksts, karsts)?

[ike]

It looks like an exception.

When speaking of the wheather, adjectivs are used for some reason.

I would also say "ārā ir slapjš" (it is humid outside).